High-strength heat-shrinkable film and preparation process thereof

Abstract

This invention discloses a high-strength heat-shrinkable film and its preparation process, relating to the field of packaging materials technology. The high-strength heat-shrinkable film is a multilayer co-extruded biaxially oriented film, comprising a surface layer and a core layer. The surface layer comprises the following raw materials: mLLDPE, PMP, quaternary graft copolymer, nano-silica, antioxidant, and erucamide; the core layer comprises the following raw materials: LDPE, EVA, polyolefin elastomer, POE-g-GMA, and antioxidant. The quaternary graft copolymer has EMH as the main chain and PMP as the grafted side chains. The preparation process of the high-strength heat-shrinkable film includes the following steps: preparing surface layer granules and core layer granules; extruding the surface layer granules and core layer granules through a co-extrusion die to form a multilayer film preform; cooling and shaping the multilayer film preform, then stretching it longitudinally and then transversely; heat-setting the stretched film and cooling it to room temperature to obtain the high-strength heat-shrinkable film. The heat-shrinkable film of this invention simultaneously achieves comprehensive performance characteristics of high transparency, high strength, high shrinkage rate, ultra-low temperature toughness, and environmental friendliness and non-toxicity.

Description

Technical Field

[0001] This invention relates to the field of packaging materials technology, specifically to a high-strength heat-shrinkable film and its preparation process. Background Technology

[0002] Heat shrink film is a type of plastic film that shrinks along its length and / or width when heated, and is widely used in product packaging, bundled packaging, and labeling. Common heat shrink film materials include polyvinyl chloride (PVC), polyethylene (PE), and polyolefin (POF). PVC film has advantages such as high transparency, good shrinkage rate, and high strength, but it also has serious drawbacks: poor environmental performance, difficult degradation after disposal, and the production of highly toxic gases such as dioxins and hydrogen chloride when burned; significant health risks, as plasticizers added to improve flexibility are prone to leach out upon contact with oils or high temperatures, making it unsuitable for food packaging; and high low-temperature brittleness, easily breaking below 0°C. While PE film is environmentally friendly, non-toxic, flexible, and resistant to low temperatures, its low transparency (typically >15%) and semi-transparent, hazy appearance make it difficult to meet the transparency requirements of high-end packaging, and its shrinkage rate is relatively low. Although POF film has good overall performance, it is more expensive and sensitive to the shrinkage process, prone to uneven shrinkage.

[0003] Therefore, developing a new type of heat shrink film that combines high transparency, high strength, high shrinkage rate, environmental friendliness, non-toxicity, and low-temperature resistance has significant market value. Summary of the Invention

[0004] The purpose of this invention is to provide a high-strength heat-shrinkable film and its preparation process. By constructing the molecular structure of the surface quaternary graft copolymer (PMP-g-EMH), the synergistic mechanism of the core LDPE / EVA / POE ternary system and the reactive compatibilizer POE-g-GMA, and the maleic anhydride-epoxy ring-opening chemical bonding network between the surface and core layers, this invention solves the comprehensive technical problem of existing heat-shrinkable films that cannot simultaneously achieve high transparency, high strength, high shrinkage rate, ultra-low temperature toughness, and environmental friendliness and non-toxicity.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] A high-strength heat-shrinkable film, wherein the high-strength heat-shrinkable film is a multilayer co-extruded biaxially oriented film, comprising a surface layer and a core layer, in parts by weight:

[0007] The surface layer comprises the following raw materials: 50-80 parts of metallocene linear low-density polyethylene (mLLDPE), 5-15 parts of polymethylpentene (PMP), 5-20 parts of quaternary graft copolymer, 1-5 parts of nano-silica, 0.1-1 parts of antioxidant 1010, and 0.1-0.5 parts of erucamide.

[0008] The core layer comprises the following raw materials: 30-60 parts of low-density polyethylene (LDPE), 20-40 parts of ethylene-vinyl acetate copolymer (EVA), 10-30 parts of polyolefin elastomer, 5-15 parts of glycidyl methacrylate grafted ethylene-octene copolymer (POE-g-GMA), and 0.1-1 parts of antioxidant 168.

[0009] The quaternary graft copolymer is a graft copolymer with ethylene-acrylate-maleic anhydride terpolymer as the main chain and polymethylpentene as the graft side chain.

[0010] On the surface layer, although polymethylpentene (PMP) possesses excellent optical properties, it is prone to phase separation when physically blended with metallocene linear low-density polyethylene (mLLDPE) due to surface energy differences. This invention grafts PMP onto the backbone of an ethylene-acrylate-maleic anhydride terpolymer (EMH) via chemical bonds, forming a graft copolymer. The maleic anhydride groups in the EMH backbone provide reactive sites, the acrylate units regulate the system's polarity, the ethylene units impart flexibility, and the grafted PMP side chains act as anchored transparent functional phases. This molecular structure eliminates interfacial defects in physical blending, allowing PMP to be uniformly dispersed in the mLLDPE matrix, achieving optical properties of ≤3% haze and ≥94% transmittance. Antioxidant 1010, with its high molecular weight and low volatility, effectively matches the high melting point of PMP (up to 235℃), effectively preventing its degradation due to thermal oxidation at high temperatures.

[0011] In terms of the core layer, this invention constructs a synergistic mechanism between the LDPE / EVA / POE ternary system and the reactive compatibilizer POE-g-GMA. Low-density polyethylene (LDPE) provides the driving force for heat shrinkage with its long-chain branched structure; the polar ester groups in ethylene-vinyl acetate copolymer (EVA) lower the glass transition temperature of the system and promote low-temperature shrinkage; polyolefin elastomer (POE) imparts flexibility to the film in extreme environments of -50°C with its glass transition temperature of approximately -60°C. Based on this, glycidyl methacrylate grafted with ethylene-octene copolymer (POE-g-GMA) with a grafting rate >1.0% is introduced. Its epoxy groups undergo a ring-opening reaction with the maleic anhydride groups of the surface layer EMH during co-extrusion, forming a chemical bond network in situ at the interface between the surface layer and the core layer, resulting in an interlayer peel strength exceeding 500 N / m, and the failure mode changing from interlayer peeling to tearing of the parent material. Meanwhile, the ethylene-octene backbone of POE-g-GMA synergistically toughens with POE in the core layer, further improving low-temperature toughness without sacrificing shrinkage performance. Antioxidant 168 not only provides active processing stability for LDPE, but also serves as a passive protection mechanism for high transparency, ensuring the final optical quality of the product by inhibiting yellowing caused by acid production from EVA thermal degradation.

[0012] Traditional multilayer heat-shrinkable films rely on physical bonding or independent adhesive layers to achieve interlayer bonding. This invention utilizes the chemical reaction between the maleic anhydride groups in the surface quaternary graft copolymer and the epoxy groups in the core POE-g-GMA to achieve interlayer chemical bonding in one step during co-extrusion. This chemically bonded network allows shrinkage stress to be uniformly transferred from the core layer to the surface layer, achieving a transverse shrinkage rate of over 55% at 100°C, with significantly better shrinkage uniformity than physical composite structures.

[0013] In addition, the nano-silica in the surface layer has both nucleation and reinforcement functions. As a nucleating agent, it promotes the formation of fine and uniform microcrystalline structures in PMP, further reducing haze. Its nanoscale effect promotes uniform stress distribution during the stretching process, increasing the surface tensile strength to over 55 MPa.

[0014] The aforementioned synergistic mechanisms work together to enable the heat shrink film of this invention to achieve a comprehensive breakthrough in high transparency, high strength, high shrinkage rate, ultra-low temperature toughness, and environmental friendliness and non-toxicity.

[0015] A process for preparing a high-strength heat-shrinkable film includes the following steps:

[0016] S100: Metallocene linear low-density polyethylene, polymethylpentene, quaternary graft copolymer, nano silica, antioxidant 1010 and erucamide are mixed evenly, heated and melt-blended, and extruded and granulated to obtain surface granules.

[0017] Low-density polyethylene, ethylene-vinyl acetate copolymer, polyolefin elastomer, glycidyl methacrylate-grafted ethylene-octene copolymer and antioxidant 168 are mixed evenly, heated and melt-blended, and then extruded and granulated to obtain core layer granules.

[0018] S200: The surface granules and core granules are extruded through a co-extrusion die to form a multilayer film preform;

[0019] S300. After cooling and shaping the obtained multilayer film preform, stretch it longitudinally first, and then stretch it transversely.

[0020] S400. The stretched film is heat-set and then cooled to room temperature to obtain a high-strength heat-shrinkable film.

[0021] The surface layer contains polymethylpentene (PMP), which has a high melting point of 235°C, requiring high processing temperatures for complete melting and plasticization. However, the maleic anhydride groups in the quaternary graft copolymer undergo accelerated hydrolysis or side reactions with antioxidants above 220°C, leading to a decrease in grafting activity. In the core layer, the ester groups of the ethylene-vinyl acetate copolymer (EVA) are prone to thermal degradation above 200°C, producing small-molecule volatiles and reducing adhesive properties. However, excessively low temperatures cannot guarantee sufficient mixing of low-density polyethylene (LDPE) and polyolefin elastomer (POE). This invention controls the surface layer granulation temperature at 180~220°C and the core layer at 160~200°C, finding a process window that balances sufficient PMP melting with maintained maleic anhydride activity, and sufficient LDPE / POE mixing with EVA thermal stability.

[0022] The ring-opening reaction between the surface maleic anhydride groups and the core epoxy groups requires sufficient activation energy, with a theoretical reaction temperature above 170°C. However, the reaction rate increases exponentially with increasing temperature. Simultaneously, the thermal degradation rate of EVA rises sharply above 210°C, and PMP undergoes significant molecular chain breakage above 230°C. This invention sets the co-extrusion die temperature to 180-220°C, seeking a critical balance between stimulating interlayer chemical bonding and inhibiting thermal degradation. This temperature window allows the reaction to complete cross-linking within seconds of molten convergence, while simultaneously controlling thermal degradation within an acceptable range.

[0023] The surface PMP crystallization temperature range is 200~230℃, requiring rapid cooling to avoid the formation of large spherulites; otherwise, haze will increase significantly. The core LDPE / EVA / POE system has a crystallization temperature range of 80~120℃; excessively rapid cooling will lead to incomplete crystallization and decreased shrinkage performance. Simultaneously, applying stretching force in the elastic state range of 50~70℃ can induce molecular chain pre-orientation, but stretching too early results in the preform not yet being shaped, while stretching too late results in the molecular chains freezing. This invention's three-stage cooling curve—rapid cooling—pre-orientation—corresponds to the three functional objectives: rapid passage of the surface layer through the crystallization zone, optimized crystallization of the core layer, and orientation induced by the elastic state.

[0024] The aforementioned cooling curve needs to simultaneously consider the surface crystallization kinetics, core crystallization kinetics, and the temperature dependence of molecular chain relaxation time. These three objectives are coupled and even conflicting on the time axis. Excessive surface quenching will prematurely lower the core temperature, causing it to miss the optimal crystallization range. This invention decouples these contradictions through a three-stage segmented control, ensuring that the thermal history requirements of each layer are compatible within a unified process path.

[0025] The final performance of heat-shrinkable film requires a balance between shrinkage rate and dimensional stability. Excessive cooling can leave residual internal stress, leading to a decrease in shrinkage rate during storage; excessive cooling, on the other hand, causes partial relaxation of the molecular chains, reducing the shrinkage rate. This invention, after heat setting, first places the film in an environment of 50-60°C for slow cooling, allowing the molecular chains to fully relax and release internal stress above the glass transition temperature. Then, it rapidly cools to room temperature at 15-20°C / second to lock the orientation structure. This slow-then-fast cooling strategy finds a process balance between the two conflicting requirements of sufficient relaxation and rapid locking, resulting in a shrinkage rate decrease of less than 0.5% after 30 days of storage at room temperature, far superior to the 2%-3% of conventional processes.

[0026] Furthermore, the preparation method of the quaternary graft copolymer includes the following steps:

[0027] Step 1: Premix ethylene-acrylate-maleic anhydride terpolymer (EMH) and polymethylpentene (PMP) evenly, and add them to the reaction system as the main material;

[0028] Step 2: When the reaction system reaches a molten state of 200~240℃, add 30%~50% of the total amount of dicumyl peroxide and 30%~50% of the total amount of antioxidant.

[0029] Step 3: After 30-120 seconds, add the remaining dicumyl peroxide (initiator) and antioxidant simultaneously;

[0030] Step 4: After 30-60 seconds, add dioctadecyl thiodipropionate to terminate the reaction.

[0031] Dicumyl peroxide (DCP) decomposes at 200–240 °C to generate alkoxy radicals. These radicals abstract hydrogen atoms from the tertiary carbons of the PMP molecular chain, forming PMP macromolecular radicals. These PMP radicals then attack the double bonds of the maleic anhydride units in the EMH backbone, forming carbon-carbon single bonds through radical addition reactions, thus grafting PMP side links onto the EMH backbone. Simultaneously, antioxidant 1010 provides hydrogen atoms to capture peroxide radicals in the system, terminating the oxidation chain reaction; antioxidant 168 decomposes hydroperoxides into stable products. Both work synergistically to inhibit thermal oxidative degradation and prevent molecular chain breakage. In the later stages of the grafting reaction, dioctadecanol thiodipropionate is added. Its thioether structure decomposes residual peroxides and captures unreacted radicals, ensuring the grafting reaction terminates promptly after reaching the target grafting rate, preventing cross-linking or over-grafting between molecular chains.

[0032] Conventional melt grafting typically involves adding the initiator all at once, leading to excessively high free radical concentrations in the initial reaction and rapid initiator decomposition. This limits the PMP grafting rate and causes excessive free radicals to initiate cross-linking side reactions, generating gels and reducing product purity. This invention employs a stepwise initiator addition: initially, 30%–50% of the total initiator is added to establish a stable free radical concentration in the reaction system; the remaining initiator is added after 30–120 seconds, allowing for a secondary grafting process after the PMP has partially melted and dispersed uniformly. This results in a more uniform distribution of grafting points along the molecular chain, significantly improving the grafting rate.

[0033] In conventional melt grafting, the free radicals generated by the initiator decomposition remain active at the end of the extruder, leading to over-grafting and cross-linking between molecular chains, resulting in increased gel content and decreased processing fluidity in the product. This invention adds dioctadecanol thiodipropionate as a terminator in the later stages of the grafting reaction. DSTP rapidly captures residual free radicals within 30-60 seconds, effectively terminating the reaction and controlling the gel content below 0.5%. The product surface is free of crystal points, ensuring the uniformity and optical quality of the subsequent film surface.

[0034] This preparation method controls the grafting process through stepwise feeding and suppresses side reactions with a terminator, thereby increasing the grafting rate while reducing the gel content. This results in a product with both a high grafting rate (16.2%~20.1%) and a low gel content (≤0.35%), providing a high-quality raw material basis for the surface properties of subsequent heat-shrinkable films.

[0035] Furthermore, the mass ratio of ethylene-acrylate-maleic anhydride terpolymer, polymethylpentene, dicumyl peroxide, and antioxidant is 100:10~30:0.2~0.5:0.2~0.3;

[0036] The amount of dioctadecyl thiodipropionate (DSTP) added is 0.5 to 1.5 times the total amount of dicumyl peroxide.

[0037] Furthermore, the antioxidants include antioxidant 1010 and antioxidant 168, with a mass ratio of antioxidant 1010 to antioxidant 168 of 1:1.5~2.3.

[0038] Further, in step S100, the core layer granules are obtained by melt blending and extrusion granulation at a temperature of 160~200℃; and the surface layer granules are obtained by melt blending and extrusion granulation at a temperature of 180~220℃.

[0039] Further, in step S200, the surface granules are extruded to form a first surface layer and a second surface layer, and the core layer is disposed between the first surface layer and the second surface layer; the thickness of the first surface layer accounts for 10% to 30% of the total film thickness; the thickness of the second surface layer accounts for 10% to 30% of the total film thickness; and the thickness of the core layer accounts for 40% to 80% of the total film thickness.

[0040] The surface PMP layer has a crystallization temperature range of 200~230℃, requiring rapid cooling during the quenching phase. The core layer LDPE / EVA / POE has a crystallization temperature range of 80~120℃, requiring relatively slow cooling to optimize crystallinity. If the surface layer is too thick, the quenching medium cannot lower the temperature of the entire surface layer below the PMP crystallization range within 1~2 seconds, resulting in insufficient crystallization and increased haze. If the core layer is too thin, its heat during the slow cooling phase is insufficient to maintain its required cooling rate, leading to incomplete crystallization and decreased shrinkage performance. The choice of thickness ratio essentially creates conditions for a gradient cooling process of quenching followed by slow cooling, allowing both layers to complete curing within their respective optimized crystallization ranges.

[0041] Chemical bonding networks transfer shrinkage stress from the core layer to the surface layer. If the surface layer is too thick, its rigidity increases, and shear concentration easily occurs at the interface during stress transfer, leading to interlayer delamination or surface cracking; if the core layer is too thin, its shrinkage driving force is insufficient, and the reactive groups on the surface cannot be fully utilized.

[0042] Furthermore, in S300, the cooling includes a rapid cooling stage and a slow cooling stage; the rapid cooling stage uses a cooling medium of 5~20°C to rapidly cool the film preform, reducing the film preform temperature to 60~80°C, with a cooling time of 1~2 seconds; the slow cooling stage uses a cooling rate of 5~15°C / second to reduce the film preform temperature from 60~80°C to 25~40°C, with the aim of optimizing the core layer crystallinity and inducing molecular chain preorientation;

[0043] In the initial stage of slow cooling, when the film blank temperature drops to 50~70℃, a longitudinal stretching force of 1.05~1.2 times is applied along the film blank's traveling direction.

[0044] The rapid cooling stage allows the surface layer to quickly pass through the crystallization temperature range of PMP, forming a microcrystalline structure. The slow cooling stage lowers the preform temperature to 50-70°C before applying pre-orientation stretching, at which point the core layer molecular chains have achieved initial orientation. Subsequently, during longitudinal stretching, the core layer molecular chains continue to extend and orient themselves based on the pre-orientation, resulting in significantly better orientation uniformity than conventional processes without pre-orientation.

[0045] Furthermore, in S300, the material is first stretched longitudinally with a stretching ratio of 3 to 5 times and a stretching temperature of 90 to 110°C; then stretched laterally with a stretching ratio of 4 to 6 times and a stretching temperature of 100 to 120°C.

[0046] The long-chain branched structure of LDPE is prone to orientation during longitudinal stretching, but requires a higher magnification ratio for full orientation during transverse stretching. If the transverse magnification ratio is too low (below 4 times), the transverse shrinkage rate of the film is difficult to reach more than 55%; if it is too high (above 6 times), the transverse strength of the film decreases excessively, affecting its performance. This differentiated magnification ratio design matches the crystallization behavior of the surface quaternary graft copolymer. The surface PMP has already formed preliminary orientation during longitudinal stretching, and the high transverse magnification ratio stretching further regularizes its microcrystalline structure, thereby improving shrinkage uniformity while ensuring transparency.

[0047] The differentiated stretching ratio of 3 to 5 times in the longitudinal direction and 4 to 6 times in the transverse direction allows the film to achieve full orientation in both directions. The transverse shrinkage rate at 100℃ reaches more than 55%, the longitudinal shrinkage rate is ≥50%, and the shrinkage uniformity is significantly better than that of the equal stretching ratio scheme.

[0048] The longitudinal stretching temperature of 90~110℃ falls within the high-elasticity range of the LDPE / EVA / POE core system, which is above the glass transition temperature and below the melting temperature. At this temperature, the molecular chains exhibit good mobility but are not completely softened. If the temperature is below 90℃, the molecular chains in the core layer lack mobility and are prone to breakage during stretching; if the temperature is above 110℃, the core layer becomes excessively softened, leading to a decrease in orientation efficiency.

[0049] The transverse stretching temperature is increased to 100~120℃. On the one hand, this is to compensate for the increased orientation resistance caused by the increased transverse stretching ratio. On the other hand, it utilizes the temperature dependence of the ring-opening reaction between the maleic anhydride groups in the surface quaternary graft copolymer and the epoxy groups in the core POE-g-GMA. The reaction rate is significantly accelerated above 100℃, which can simultaneously complete interlayer chemical bonding during transverse stretching, thus achieving the integration of stretching orientation and interfacial reaction.

[0050] Due to proper temperature control during the stretching process, the microcrystalline structure of the surface PMP film was not damaged, resulting in a haze of ≤3% and a light transmittance of ≥94%, which is superior to conventional PE film.

[0051] Furthermore, in S400, the heat setting temperature is 80~100℃ and the heat setting time is 5~15 seconds. After heat setting, the temperature is first reduced to 50~60℃ at a cooling rate of 5~10℃ / second to release the thermal stress generated after heat setting. Then, the temperature is cooled to room temperature at a cooling rate of 15~20℃ / second.

[0052] Heat setting itself eliminates the internal stress generated during stretching by treating the film at high temperature for a short time. However, after heat setting, the film will still generate new internal stress due to thermal shrinkage during the cooling process from the setting temperature. The slow cooling stage of this invention forms a sequential division of labor between heat setting and stress release. Heat setting is responsible for eliminating the internal stress generated by stretching, while the slow cooling stage is responsible for eliminating the thermal stress generated during the cooling process. The two complement each other to minimize the internal stress of the film.

[0053] Compared with the prior art, the beneficial effects of the present invention are:

[0054] 1. This invention grafts polymethylpentene onto the main chain of an ethylene-acrylate-maleic anhydride terpolymer via chemical bonds to form a quaternary graft copolymer. This eliminates the phase separation problem during the physical blending of PMP and metallocene linear low-density polyethylene at the molecular level, allowing PMP to be uniformly dispersed in the matrix. This achieves a haze of ≤3% and a light transmittance of ≥94%, reaching or even surpassing the optical level of existing high-end POF films. At the same time, it avoids the problem of the optical performance of PVC films deteriorating at low temperatures.

[0055] 2. This invention achieves a tensile strength of over 55 MPa while maintaining excellent flexibility through the dual effects of nucleation and reinforcement of the surface nano-silica and the synergistic toughening of the core LDPE / EVA / POE ternary system. Specifically, the polyolefin elastomer (POE), with its glass transition temperature of approximately -60°C, forms a dispersed toughening phase within the continuous LDPE phase, synergistically working with the interfacial compatibilization effect of EVA to achieve a balance between high strength and high toughness.

[0056] 3. This invention utilizes the synergistic effect of a ternary structure of LDPE / EVA / POE in the core layer, enabling the film to achieve a transverse shrinkage rate of ≥56% and a longitudinal shrinkage rate of ≥50% at 100℃, with a shrinkage initiation temperature 25~40℃ lower than that of traditional POF films. Simultaneously, the chemical bonding network between the quaternary graft copolymer in the surface layer and the POE-g-GMA in the core layer ensures that shrinkage stress is uniformly transferred from the core layer to the surface layer, resulting in significantly better shrinkage uniformity than physical composite structures.

[0057] 4. This invention utilizes the low glass transition temperature of POE in the core layer to form a synergistic system with LDPE and EVA, which consists of continuous phase support, dispersed phase toughening, and interface compatibilization. This allows the film to maintain its flexibility and prevent brittleness in extreme environments such as -50℃, meeting the stringent requirements of frozen foods, low-temperature storage, and transportation in cold regions.

[0058] 5. Both the surface and core layers of this invention use a pure polyolefin system, which contains no chlorine or plasticizers, and the processing does not produce toxic gases. Furthermore, the all-polyolefin system is easy to recycle and regenerate. Detailed Implementation

[0059] The present invention will now be further described.

[0060] Example 1

[0061] The mass ratio of ethylene-acrylate-maleic anhydride terpolymer, polymethylpentene, dicumyl peroxide, and composite antioxidant is 100:20:0.33:0.26.

[0062] The amount of dioctadecyl thiodipropionate added is equal to the total amount of dicumyl peroxide.

[0063] The compound antioxidant includes antioxidant 1010 and antioxidant 168, with a mass ratio of antioxidant 1010 to antioxidant 168 of 1:2.

[0064] Preparation of quaternary graft copolymers:

[0065] The co-rotating parallel twin-screw extruder with a length-to-diameter ratio of 52:1, a screw diameter of 65mm, a rotation speed of 500~600rpm, and a motor power of approximately 132kW was used for reactive extrusion. The Kronwell 65B model was used.

[0066] After EMH and PMP are premixed evenly in a high-speed mixer, they are continuously metered and fed into the barrel from the first main feed port of the extruder via the main feeding system. The extruder is equipped with 10 independent temperature control zones (including the die head zone) along the material flow direction, and the temperature gradient of each zone is set as follows:

[0067] Feeding and conveying zone (zones 1-2): 180℃, 190℃ – to rapidly heat the material and initiate melting;

[0068] Melting and mixing zone (zones 3-4): 200℃, 210℃ — the material is fully melted and PMP is softened and dispersed;

[0069] Reaction zones (zones 5-7): 220℃, 230℃, 230℃ — DCP decomposes, grafting reaction occurs;

[0070] Deviation zone (zones 8-9): 210℃, 200℃ — Vacuum devolatilization to remove small molecule byproducts;

[0071] Die head area (zone 10): 210℃ - pressure build-up extrusion.

[0072] When the reaction system reaches 220℃ (initial stage of zone 5), add 40% of the total DCP and 40% of the total composite antioxidant (DCP solution concentration 18wt%, solvent: acetone) through the first liquid feed port. After 80 seconds (when the material reaches the middle stage of zone 6), add the remaining 60% of DCP and composite antioxidant through the second liquid feed port.

[0073] A vacuum devouring port is installed at the end of the extruder, with a vacuum degree of 0.06~0.08MPa, to remove unreacted initiator decomposition products, solvents, and small molecule byproducts. 45 seconds after the second feeding (when the material reaches zone 8), an equal amount of DSTP terminator (DSTP prepared as a 10~15wt% acetone solution) is added through the third liquid feed port (near the die head) to terminate unreacted free radicals.

[0074] After the extruded strip is cooled in a water bath (water temperature 20~30℃) and air-dried (blown with compressed air), it is pelletized by a pelletizer (the pelletizing speed is linked to the extrusion speed) to obtain quaternary graft copolymer particles.

[0075] Example 2

[0076] The mass ratio of ethylene-acrylate-maleic anhydride terpolymer, polymethylpentene, dicumyl peroxide, and composite antioxidant is 100:10:0.2:0.2;

[0077] The amount of dioctadecyl thiodipropionate added is 0.5 times the total amount of dicumyl peroxide.

[0078] The compound antioxidant includes antioxidant 1010 and antioxidant 168, with a mass ratio of antioxidant 1010 to antioxidant 168 of 1:1.5.

[0079] Preparation of quaternary graft copolymers:

[0080] The same co-rotating parallel twin-screw extruder as in Example 1 was used.

[0081] After EMH and PMP are premixed evenly, they are fed into the extruder through the first feeding port of the main feeding system. The temperature gradient of the 10 temperature zones of the extruder is set as follows:

[0082] Feeding and conveying zone (zones 1-2): 160℃, 170℃;

[0083] Melting and mixing zone (zones 3-4): 180℃, 190℃;

[0084] Reaction zones (zones 5-7): 200℃, 210℃, 210℃;

[0085] Deviation zones (zones 8-9): 190℃, 180℃;

[0086] Nose area (zone 10): 190℃.

[0087] When the reaction system reaches 200℃ (initial stage of zone 5), 30% of the total amount of dicumyl peroxide and 30% of the total amount of composite antioxidant (DCP solution concentration 18wt%, solvent: acetone) are added through the first liquid feed port. After 30 seconds (when the material reaches the middle stage of zone 6), the remaining 70% of dicumyl peroxide and composite antioxidant are added through the second liquid feed port.

[0088] The vacuum degree at the vacuum devouring port at the end of the extruder is 0.05~0.07MPa. 30 seconds after the second feeding (when the material has traveled to zone 8), add 0.5 times the total amount of dicumyl peroxide dioctyl thiodipropionate through the third liquid feed port to terminate the reaction.

[0089] The extruded strip is cooled in a water bath, air-dried, and then pelletized to obtain a quaternary graft copolymer.

[0090] Example 3

[0091] The mass ratio of ethylene-acrylate-maleic anhydride terpolymer, polymethylpentene, dicumyl peroxide, and composite antioxidant is 100:30:0.5:0.3.

[0092] The amount of dioctadecyl thiodipropionate added is 1.5 times the total amount of dicumyl peroxide.

[0093] The compound antioxidant includes antioxidant 1010 and antioxidant 168, with a mass ratio of antioxidant 1010 to antioxidant 168 of 1:2.3.

[0094] Preparation method of quaternary graft copolymers:

[0095] The same co-rotating parallel twin-screw extruder as in Example 1 was used.

[0096] After EMH and PMP are premixed evenly, they are fed into the extruder through the first feeding port of the main feeding system. The temperature gradient of the 10 temperature zones of the extruder is set as follows:

[0097] Feeding and conveying zones (zones 1-2): 190℃, 200℃;

[0098] Melting and mixing zone (zones 3-4): 210℃, 220℃;

[0099] Reaction zones (zones 5-7): 230℃, 240℃, 240℃;

[0100] Deviation zones (zones 8-9): 220℃, 210℃;

[0101] Nose area (zone 10): 210℃.

[0102] When the reaction system reaches 240℃ (initial stage of zone 6), 50% of the total amount of dicumyl peroxide and 50% of the total amount of composite antioxidant (DCP solution concentration 18wt%, solvent: acetone) are added through the first liquid feed port. 120 seconds later (when the material reaches the middle stage of zone 7), the remaining 50% of dicumyl peroxide and composite antioxidant are added through the second liquid feed port.

[0103] The vacuum degree at the vacuum devouring port at the end of the extruder is 0.07~0.09MPa. 60 seconds after the second feeding (when the material has traveled to zone 9), add 1.5 times the total amount of dicumyl peroxide dithiodipropionate dioctadecyl ester through the third liquid feed port to terminate the reaction.

[0104] The extruded strip is cooled in a water bath, air-dried, and then pelletized to obtain a quaternary graft copolymer.

[0105] Comparative Example 1

[0106] Steps 2 and 3 are replaced by adding dicumyl peroxide and an antioxidant when the reaction system reaches a molten state at 240°C. Step 4 is omitted, and dioctadecyl thiodipropionate is not added.

[0107] Other parameters and steps 1 are the same as in Example 1.

[0108] Comparative Example 2

[0109] Steps 2 and 3 are replaced by adding dicumyl peroxide and an antioxidant when the reaction system reaches a molten state at 240°C.

[0110] The remaining steps and parameters are the same as in Example 1.

[0111] Comparative Example 3

[0112] Step 4 is omitted, and dioctadecyl thiodipropionate is not added. The remaining steps and parameters are the same as in Example 1.

[0113] Comparative Example 4

[0114] The mass ratio of antioxidant 1010 to antioxidant 168 during heating was adjusted to 1:0.5. The remaining steps and parameters were the same as in Example 1.

[0115] Comparative Example 5

[0116] The mass ratio of antioxidant 1010 to antioxidant 168 during heating was adjusted to 1:3. The remaining steps and parameters were the same as in Example 1.

[0117] Comparative Example 6

[0118] No antioxidants were added. The remaining steps and parameters were the same as in Example 1.

[0119] Comparative Example 7

[0120] Antioxidant 1010 was used alone. The remaining steps and parameters were the same as in Example 1.

[0121] Comparative Example 8

[0122] Antioxidant 168 was used alone. The remaining steps and parameters were the same as in Example 1.

[0123] Comparative Example 9

[0124] The terminator was replaced with triphenyl phosphite. The remaining steps and parameters were the same as in Example 1.

[0125] Comparative Example 10

[0126] The melting reaction temperature was lowered to 180°C. The remaining steps and parameters were the same as in Example 1.

[0127] Comparative Example 11

[0128] The melting reaction temperature was raised to 260°C. The remaining steps and parameters were the same as in Example 1.

[0129] The properties of the quaternary graft copolymers prepared by the methods in Examples 1-3 and Comparative Examples 1-11 are shown in Table 1.

[0130] Table 1. Performance of the quaternary graft copolymers prepared by the methods of Examples 1-3 and Comparative Examples 1-11

[0131]

[0132] As shown in Table 1, the grafting rates of Examples 1-3 were 18.5%, 16.2%, and 20.1%, respectively, all significantly higher than those of all comparative examples. In Comparative Example 1, the initial free radical concentration was too high due to the one-time addition of the initiator, causing rapid decomposition of the initiator. Most free radicals terminated coupling or initiated cross-linking side reactions before participating in grafting, resulting in a grafting rate of only 8.2%. Comparative Example 2 achieved a grafting rate of 9.5%, a slight improvement but still far lower than the examples, indicating that the initial high initiator concentration cannot be solved by using only a terminator. Comparative Example 3 achieved a grafting rate of 12.4%, a significant improvement over the one-time addition, demonstrating that stepwise addition allows for the continuous generation of PMP macromolecular free radicals over a longer period, resulting in a more uniform distribution of grafting points. However, free radicals that were not terminated in time still consumed some PMP side chains or initiated cross-linking, leading to a grafting rate that was not optimal.

[0133] Comparative Example 9 showed a grafting rate of 13.2%, which was still lower than that of the Example. Triphenyl phosphite, as a phosphite terminator, has a lower efficiency in capturing free radicals than DSTP and may inhibit the grafting reaction to some extent.

[0134] Comparative Example 5 had a grafting rate of 14.2%, the highest among examples, but the excessively high proportion of antioxidant 168 would over-consume initiator free radicals, resulting in a grafting rate lower than Examples 1-3. Comparative Example 10 had a grafting rate of only 6.8%, with the low temperature leading to insufficient DCP decomposition rate and low free radical concentration. Comparative Example 11 had a grafting rate of 8.5%, although high temperature accelerated initiator decomposition, thermal degradation of PMP and EMH dominated, resulting in a decrease in the grafting rate.

[0135] Stepwise feeding and appropriate temperature are the foundation for improving the grafting rate, while the DSTP terminator precisely quenches residual free radicals in the later stage of the reaction, preventing the grafting points from being destroyed. The synergy of these three factors increases the grafting rate to over 16%.

[0136] The gel contents of Examples 1-3 were 0.28%, 0.22%, and 0.35%, respectively. Comparative Example 1 had a gel content as high as 3.65%, indicating excessive accumulation of free radicals leading to extensive cross-linking. Comparative Example 3 had a gel content of 1.85%, which, although lower than the single-feed method, was still significantly higher than the Examples, demonstrating that stepwise feeding can reduce cross-linking but cannot completely inhibit it; a terminator is necessary. Comparative Example 2 had a gel content of 0.52%, slightly higher than the Examples, indicating that while a terminator in a single-feed method can quench later-stage free radicals, the cross-linked structures already formed in the early stages cannot be reversed.

[0137] Comparative Example 6 had a gel content of 4.85%, and the free radicals generated by thermal oxidation degradation accelerated cross-linking. Comparative Example 11 had a gel content of 1.95%, and high temperature intensified thermal oxidation and cross-linking. Comparative Example 10 had a gel content of 0.45%, which, although low, resulted in a severely insufficient grafting rate, rendering it without practical application value. Comparative Example 9 had a gel content of 0.68%, higher than the examples, confirming that DSTP has superior termination efficiency.

[0138] The stepwise feeding reduced the peak concentration of free radicals in the early stage of the reaction, and the DSTP terminator promptly removed residual free radicals in the later stage of the reaction. The combination of the two reduced the gel content to below 0.3%, while the antioxidant compound further inhibited the crosslinking induced by thermal oxidation.

[0139] Melt index reflects the molecular weight and molecular weight distribution of the product, and directly affects the flowability and film stability during subsequent film extrusion processing.

[0140] The melt indices of Examples 1-3 were 4.2, 5.1, and 3.5 g / 10 min, respectively, all within the ideal processing window. Example 2, due to its lower reaction temperature, smaller total amount of initiator, and lowest degree of molecular chain degradation, had the highest melt index, reaching 5.1 g / 10 min.

[0141] Comparative Example 1: Melt index only 1.8 g / 10 min, excessive crosslinking led to a significant increase in molecular weight and deterioration in flowability. Comparative Example 3: Melt index 2.6 g / 10 min, still relatively low. Comparative Example 2: Melt index 3.5 g / 10 min, close to the lower limit. Comparative Example 6: Melt index 1.2 g / 10 min, thermal degradation and crosslinking coexisted, and the molecular weight distribution broadened. Comparative Example 11: Melt index 2.2 g / 10 min, crosslinking and degradation occurred simultaneously at high temperature.

[0142] Comparative Example 5 had a melt index of 3.1 g / 10 min, slightly lower than the example, possibly due to excessive consumption of the initiator by 168, resulting in a higher molecular weight. Comparative Example 10 had a melt index of 3.8 g / 10 min, which, while having acceptable flowability, had an excessively low grafting rate.

[0143] The combination of stepwise feeding and terminator allows for controllable molecular weight distribution, while the antioxidant compound inhibits degradation and avoids excessive cross-linking, thereby stabilizing the melt index within a suitable processing range and ensuring the smooth progress of subsequent granulation and co-extrusion processes.

[0144] The yellow index reflects the degree of oxidation and discoloration of the product during high-temperature processing. The lower the value, the closer the appearance is to white or colorless and transparent.

[0145] The yellow indices of Examples 1-3 were 2.1, 1.9, and 2.5, respectively, all at very low levels. Comparative Example 4 had a yellow index as high as 7.2; when the 168 ratio was insufficient, 1010 reacted with peroxide free radicals to generate quinone-like colored substances, leading to severe yellowing. For the same reason, Comparative Example 7 had a yellow index of 6.8. Comparative Example 6 had a yellow index of 12.5, indicating severe thermal oxidative degradation, with the product being deep yellow. Comparative Example 11 had a yellow index of 8.5, indicating that high temperature intensified oxidation. Comparative Example 1 had a yellow index of 5.6, with residual free radicals also causing discoloration.

[0146] Comparative Example 8 had a yellow index of 2.8, lower than the Example, but the grafting rate was only 9.8%, so sacrificing reactivity for appearance is not advisable. Comparative Example 5 had a yellow index of 3.5, still higher than the Example. Comparative Example 9 had a yellow index of 3.5; triphenyl phosphite itself has some anti-yellowing effect, but it is not as good as the synergistic effect of DSTP and antioxidants.

[0147] Antioxidant 1010 and 168 are compounded in a ratio of 1:1.5~2.3, which allows 168 to preferentially decompose hydroperoxides, reducing the consumption of 1010 and the generation of colored byproducts. At the same time, the DSTP terminator captures residual free radicals, together controlling the yellow index below 2.5, ensuring the high whiteness of the graft copolymer, which is beneficial to the high transparency of the subsequent film.

[0148] Maleic anhydride retention rate refers to the proportion of maleic anhydride groups that remain active in the EMH backbone after processing. These groups are key reaction sites for subsequent chemical bonding between the surface and core layers.

[0149] The maleic anhydride retention rates in Examples 1-3 were 92%, 94%, and 88%, respectively, all at very high levels. Example 6 had a retention rate of only 45%, indicating that thermal oxidation caused the maleic anhydride to undergo ring-opening hydrolysis or react with degradation products. Comparative Example 11 had a retention rate of 58%, indicating that high temperature directly led to the thermal decomposition of maleic anhydride. Comparative Example 10 had a retention rate of 75%, indicating that although the temperature was low, the grafting reaction was incomplete, and the residual initiator may have slowly consumed the maleic anhydride. Comparative Example 1 had a retention rate of 68%, indicating that free radicals excessively attacked the maleic anhydride double bonds. Comparative Example 3 had a retention rate of 82%, lower than the examples, because the free radicals were not terminated in time in the later stages.

[0150] Comparative Example 7: Retention rate 72%, antioxidant 1010 was unable to effectively inhibit the thermal oxidation of maleic anhydride. Comparative Example 8: Retention rate 70%, antioxidant 168 provided insufficient protection against free radical attack on maleic anhydride. Comparative Example 9: Retention rate 80%, lower than DSTP. Comparative Example 5: Retention rate 85%, close to the example, but the grafting rate was low.

[0151] The synergistic effect of the antioxidant compound and the DSTP terminator not only inhibits the damage of maleic anhydride to thermal oxidation, but also avoids the attack of free radicals on its double bonds, so that the retention rate of maleic anhydride is kept stable at more than 88%, providing sufficient active sites for subsequent interlayer chemical bonding.

[0152] Example 4

[0153] A high-strength heat-shrinkable film, wherein the high-strength heat-shrinkable film is a multilayer co-extruded biaxially oriented film, comprising a surface layer and a core layer.

[0154] The grafting rate of glycidyl methacrylate-grafted ethylene-octene copolymer is >1.0% (Jia Yi Rong® SOG-02).

[0155] The preparation process of high-strength heat-shrinkable film, using a Putong FOCI-05 film biaxial stretching apparatus, includes the following steps:

[0156] S100, surface granules: 650g mLLDPE, 100g PMP, 120g quaternary graft copolymer (prepared by the method in Example 1), 30g nano silica, 5g antioxidant 1010, and 2g erucamide are mixed evenly, melt-blended at 200°C, and then extruded and granulated.

[0157] Core layer granules: Mix 450g LDPE, 300g EVA, 200g POE, 100g POE-g-GMA, and 168.5g antioxidant evenly, melt blend at 180℃, and then extrude and granulate.

[0158] S200: The surface granules and core granules are extruded through a co-extrusion die (die temperature 200℃) to form a three-layer preform: a first surface layer, a core layer, and a second surface layer. The thickness of the first surface layer accounts for 20% of the total film thickness, the thickness of the second surface layer accounts for 20%, and the thickness of the core layer accounts for 60%.

[0159] S300, Cooling: In the rapid cooling stage, the preform is rapidly cooled using a 12°C cooling medium (air) to reduce its temperature to 70°C in 1.5 seconds. In the slow cooling stage, the preform is cooled from 70°C to 30°C at a rate of 10°C / second. In the initial stage of slow cooling, when the preform temperature drops to 60°C, a longitudinal stretching force of 1.1 times is applied along the direction of travel.

[0160] Stretching: First, stretch longitudinally at a stretch ratio of 4 times and a stretching temperature of 100℃; then stretch transversely at a stretch ratio of 5 times and a stretching temperature of 110℃.

[0161] S400, heat setting: temperature 90℃, time 10 seconds. After heat setting, first cool to 55℃ at a cooling rate of 8℃ / second, then cool to room temperature (25℃) at a cooling rate of 18℃ / second to obtain a high-strength heat-shrinkable film.

[0162] Example 5

[0163] A high-strength heat-shrinkable film, wherein the high-strength heat-shrinkable film is a multilayer co-extruded biaxially oriented film, comprising a surface layer and a core layer.

[0164] The grafting rate of glycidyl methacrylate-grafted ethylene-octene copolymer is >1.0% (Jia Yi Rong® SOG-02).

[0165] The preparation process of high-strength heat-shrinkable film, using a Putong FOCI-05 film biaxial stretching apparatus, includes the following steps:

[0166] S100, surface granules: mLLDPE 500g, PMP 50g, quaternary graft copolymer (prepared by the method in Example 1) 50g, nano silica 10g, antioxidant 10101g, erucamide 1g, melt blended and extruded granulated at 180°C.

[0167] Core layer granules: LDPE 300g, EVA 200g, POE 100g, POE-g-GMA 50g, antioxidant 168 1g, melt blended and extruded at 160℃.

[0168] S200, co-extrusion die temperature 180℃. First surface layer thickness 10%, second surface layer thickness 10%, core layer thickness 80%.

[0169] S300, rapid cooling: cooling medium at 5℃, reducing the preform temperature to 60℃ in 1 second; slow cooling: cooling at a rate of 5℃ / second to 25℃. When the preform temperature drops to 50℃, apply 1.05 times the longitudinal stretching force.

[0170] Stretching: First, stretch longitudinally at a stretch ratio of 3 times and a stretching temperature of 90℃; then stretch transversely at a stretch ratio of 4 times and a stretching temperature of 100℃.

[0171] S400, heat setting: temperature 80℃, time 5 seconds. After heat setting, first cool down to 50℃ at 5℃ / second, then cool down to room temperature 22℃ at 15℃ / second.

[0172] Example 6

[0173] A high-strength heat-shrinkable film, wherein the high-strength heat-shrinkable film is a multilayer co-extruded biaxially oriented film, comprising a surface layer and a core layer.

[0174] The grafting rate of glycidyl methacrylate-grafted ethylene-octene copolymer is >1.0% (Jia Yi Rong® SOG-02).

[0175] The preparation process of high-strength heat-shrinkable film, using a Putong FOCI-05 film biaxial stretching apparatus, includes the following steps:

[0176] S100, surface granules: mLLDPE 800g, PMP 150g, quaternary graft copolymer (prepared by the method in Example 1) 200g, nano silica 50g, antioxidant 1010 10g, erucamide 5g, melt blended and extruded granulated at 220℃.

[0177] Core layer granules: LDPE 600g, EVA 400g, POE 300g, POE-g-GMA 150g, antioxidant 168 10g, melt blended and extruded at 200℃.

[0178] S200, co-extrusion die temperature 220℃. First surface layer thickness 30%, second surface layer thickness 30%, core layer thickness 40%.

[0179] S300, rapid cooling: cooling medium 20℃, reducing the preform temperature to 80℃, cooling time 2 seconds; slow cooling: cooling rate of 15℃ / second to 40℃. When the preform temperature drops to 70℃, apply 1.2 times the longitudinal stretching force.

[0180] Stretching: First, stretch longitudinally at a stretch ratio of 5 times and a stretching temperature of 110℃; then stretch transversely at a stretch ratio of 6 times and a stretching temperature of 120℃.

[0181] S400, heat setting: temperature 100℃, time 15 seconds. After heat setting, first cool down to 60℃ at 10℃ / second, then cool down to room temperature 28℃ at 20℃ / second.

[0182] Comparative Example 12

[0183] The surface layer does not contain quaternary graft copolymer, but contains an additional 20g PMP and 100g mLLDPE, otherwise it is the same as in Example 4.

[0184] Comparative Example 13

[0185] The surface layer is replaced with an ungrafted EMH and PMP physical blend instead of the quaternary graft copolymer, otherwise the same as in Example 4.

[0186] Comparative Example 14

[0187] No POE-g-GMA was added to the core layer, and the LDPE was increased to 55g. Other aspects were the same as in Example 4.

[0188] Comparative Example 15

[0189] The surface granulation temperature is 250°C, the core granulation temperature is 220°C, and other conditions are the same as in Example 4.

[0190] Comparative Example 16

[0191] The co-extrusion die temperature was 230°C, and other conditions were the same as in Example 4.

[0192] Comparative Example 17

[0193] In step S300, natural cooling is used, and the rest is the same as in Example 4.

[0194] Comparative Example 18

[0195] In step S300, the longitudinal stretching force is omitted during the cooling process (no pre-oriented stretching), and the rest is the same as in Example 4.

[0196] Comparative Example 19

[0197] In step S300, the material is first stretched laterally, then stretched longitudinally, and the rest is the same as in Example 4.

[0198] Comparative Example 20

[0199] In step S300, the longitudinal stretch is doubled and the transverse stretch is doubled, and the rest is the same as in Example 4.

[0200] Comparative Example 21

[0201] In step S400, after heat setting is completed, slow cooling is omitted and the temperature is directly cooled to room temperature at a cooling rate of 15~20℃ / second, otherwise the same as in Example 4.

[0202] Comparative Example 22

[0203] The surface layer uses antioxidant 168, the core layer uses antioxidant 1010, and the rest is the same as in Example 4.

[0204] Performance comparison tests were conducted based on Examples 4-6 and Comparative Examples 12-22, and the test results are shown in Table 2. The performance test methods are as follows:

[0205] Haze: ASTM D1003, in %

[0206] Light transmittance: ASTM D1003, unit: %

[0207] Tensile strength: ASTM D882, unit MPa;

[0208] Transverse shrinkage rate at 100℃: Measured after immersing a 10cm×10cm sample in a 100℃ glycerol bath for 5 seconds, unit: %

[0209] Brittle fracture rate at -50℃: ASTM D1790, in %

[0210] Interlayer peel strength: ASTM D903, unit N / m;

[0211] Shrinkage attenuation rate: The percentage decrease in lateral shrinkage at 100℃ after 30 days of storage at room temperature, expressed as % .

[0212] Melt flow index (core layer granules): GB / T 3682-2018, 190℃ / 2.16kg, unit g / 10min.

[0213] Table 2 Performance test results of heat shrink films prepared by the methods of Examples 4-6 and Comparative Examples 12-22

[0214]

[0215] As shown in Table 2, Examples 4-6 have a haze of 2.4-2.8%, a light transmittance of 93.5-94.8%, a transverse shrinkage rate of 56-61% at 100℃, a transverse shrinkage rate of 50-54% at 100℃, a brittle fracture rate of 0 at -50℃, an interlayer peel strength of 520-610 N / m, and a shrinkage attenuation rate of 0.3-0.5%.

[0216] Comparative Example 12 showed a haze of 7.8%, light transmittance of 88.5%, and tensile strength of 44.2 MPa, which were significantly worse than those of Example 4. This indicates that physical blending cannot solve the phase separation between PMP and mLLDPE, leading to a significant decrease in optical and mechanical properties.

[0217] Comparative Example 13 had a haze of 6.5% and a peel strength of 410 N / m, which were still far lower than those of Example 4. This demonstrates that simply blending EMH and PMP without chemical grafting cannot form an effective main-chain-side-chain structure, interface defects still exist, and there is no chemical bond between the maleic anhydride groups and PMP, affecting the efficiency of interlayer reaction.

[0218] Comparative Example 14, without POE-g-GMA, had an interlaminar peel strength of only 225 N / m, physical bonding, and a brittle fracture rate as high as 35% at -50℃, indicating a lack of chemical bonding and synergistic toughening with POE-g-GMA, resulting in severely insufficient low-temperature toughness.

[0219] Comparative Example 15 (surface layer 250℃ / core layer 220℃) showed an increase in haze to 5.8%, a decrease in strength to 48.2 MPa, a decrease in transverse shrinkage to 48%, a decrease in longitudinal shrinkage to 43%, and 10% brittleness at -50℃. This was due to excessively high surface temperatures leading to maleic anhydride hydrolysis and PMP degradation; and excessively high core temperatures causing EVA thermal degradation, producing acidic substances that disrupt interlayer bonds and reduce shrinkage performance. Example 4 controlled the surface temperature at 200℃ and the core temperature at 180℃, achieving a balance between sufficient PMP melting and maintained maleic anhydride activity, and between EVA thermal stability and thorough mixing of LDPE / POE.

[0220] Comparative Example 16, co-extruded at 230℃, exhibited a haze of 3.5%, a peel strength of 310 N / m, a transverse shrinkage rate of 49%, and a longitudinal and transverse shrinkage rate reduced to 44%. At -50℃, the brittleness was 15%. Above 220℃, EVA thermal degradation intensified, acidic products inhibited the ring-opening reaction, and PMP began to degrade, leading to a comprehensive decline in optical and mechanical properties.

[0221] Comparative Example 17: Natural cooling resulted in a haze of 5.2% and a shrinkage rate of 50%. Insufficient natural cooling rate led to incomplete PMP crystallization (large spherulites), resulting in increased haze; simultaneously, excessively rapid core crystallization reduced shrinkage performance.

[0222] Comparative Example 18, without pre-orientation stretching, had a lateral shrinkage rate of 53%, lower than the 58% of Example 4. This indicates that applying a stretching force of 1.1 times during the initial slow cooling phase can induce pre-orientation of the core layer molecular chains, providing an orientation basis for subsequent longitudinal stretching and improving the final shrinkage rate.

[0223] Comparative Example 19, with the stretching proceeding laterally followed by longitudinal stretching, exhibited a haze of 3.0%, a shrinkage rate of 52%, and a strength of 52.5 MPa. When stretching laterally first, the surface PMP microcrystals had not yet undergone longitudinal orientation. High-ratio stretching laterally easily disrupted the regularity of the microcrystals, leading to a slight increase in haze and a decrease in shrinkage. The longitudinal-to-lateral stretching sequence of this invention is more conducive to the stepwise orientation of molecular chains.

[0224] Comparative Example 20, with an equal stretching ratio, showed a shrinkage rate of only 41% and a strength of 46.5 MPa. The long-chain branched structure of LDPE requires a higher transverse stretching ratio to achieve sufficient orientation; at the equal stretching ratio, insufficient transverse orientation leads to a significant reduction in shrinkage. Simultaneously, PMP microcrystals exhibit poor regularity and increased haze at low stretching ratios. Example 4, with its differentiated stretching ratios of 4 times longitudinally and 5 times transversely, achieved a shrinkage rate of ≥58% and high strength.

[0225] Comparative Example 21, without slow cooling, underwent direct rapid cooling, resulting in a shrinkage attenuation rate as high as 2.4%, while Example 4 only experienced a 0.4% shrinkage attenuation rate. Direct rapid cooling froze the thermal stress generated after heat setting, which was gradually released during storage, leading to a decrease in shrinkage. Example 4 first underwent slow cooling (8°C / second to 55°C) to fully relax the molecular chains and release stress, followed by rapid cooling to lock the orientation, resulting in excellent dimensional stability.

[0226] Comparative Example 22 (surface layer 168 / core layer 1010) exhibited a haze of 3.2%, light transmittance of 92.6%, strength of 54.2 MPa, and shrinkage attenuation of 1.2%. The surface layer using 168 (phosphite) provided insufficient protection against PMP at high temperatures, leading to slight degradation (increased haze); the core layer using 1010 could not effectively suppress the acidic substances produced by the thermal degradation of EVA, resulting in increased shrinkage attenuation. Example 4, with its matched design of a surface layer using 1010 and a core layer using 168, balanced high-temperature stability and resistance to yellowing, achieving optimal performance.

[0227] In summary, the quaternary graft copolymer in this invention is an irreplaceable means to solve the phase separation of PMP / mLLDPE and achieve high transparency and high peel strength; simple physical blending cannot achieve the same effect. The stratified granulation temperature and co-extrusion temperature are critical conditions for balancing the thermal properties and reactivity of each component; deviations will lead to a comprehensive decline in optical, mechanical, and shrinkage properties. The three-stage cooling process of rapid cooling-slow cooling-pre-orientation matches the surface PMP crystallization kinetics, core layer crystallization kinetics, and molecular chain relaxation time, achieving a balance between low haze (≤3%) and high shrinkage (transverse ≥55%, longitudinal ≥50%). The longitudinal-to-transverse stretching sequence and differentiated stretching ratios (3~5 times longitudinally / 4~6 times transversely) fully utilize the branched structure of LDPE and the orientation characteristics of PMP microcrystals, resulting in a shrinkage rate of over 55% and uniform shrinkage. The slow cooling followed by rapid cooling after heat setting effectively releases thermal stress, resulting in a shrinkage attenuation rate of less than 0.5%, which is far superior to conventional processes.

Claims

1. A high-strength heat-shrinkable film, characterized in that, The high-strength heat-shrinkable film is a multi-layer co-extruded biaxially oriented film, comprising a surface layer and a core layer, in parts by weight: The surface layer comprises the following raw materials: Metallocene linear low-density polyethylene 50-80 parts, polymethylpentene 5-15 parts, quaternary graft copolymer 5-20 parts, nano silica 1-5 parts, antioxidant 1010 0.1-1 parts, erucamide 0.1-0.5 parts; The core layer comprises the following raw materials: 30-60 parts of low-density polyethylene, 20-40 parts of ethylene-vinyl acetate copolymer, 10-30 parts of polyolefin elastomer, 5-15 parts of glycidyl methacrylate grafted ethylene-octene copolymer, and 0.1-1 parts of antioxidant 168. The quaternary graft copolymer is a graft copolymer with ethylene-acrylate-maleic anhydride terpolymer as the main chain and polymethylpentene as the graft side chain.

2. A preparation process for the high-strength heat-shrinkable film as described in claim 1, characterized in that, Includes the following steps: S100: Metallocene linear low-density polyethylene, polymethylpentene, quaternary graft copolymer, nano silica, antioxidant 1010 and erucamide are mixed evenly, heated and melt-blended, and extruded and granulated to obtain surface granules. Low-density polyethylene, ethylene-vinyl acetate copolymer, polyolefin elastomer, glycidyl methacrylate-grafted ethylene-octene copolymer and antioxidant 168 are mixed evenly, heated and melt-blended, and then extruded and granulated to obtain core layer granules. S200: The surface granules and core granules are extruded through a co-extrusion die to form a multilayer film preform; S300. After cooling and shaping the obtained multilayer film preform, stretch it longitudinally first, and then stretch it transversely. S400. The stretched film is heat-set and then cooled to room temperature to obtain a high-strength heat-shrinkable film.

3. The preparation process according to claim 2, characterized in that, The preparation method of the quaternary graft copolymer includes the following steps: Step 1: Premix the ethylene-acrylate-maleic anhydride terpolymer with polymethylpentene until homogeneous, and add it to the reaction system as the main material; Step 2: When the reaction system reaches a molten state of 200~240℃, add 30%~50% of the total amount of dicumyl peroxide and 30%~50% of the total amount of antioxidant. Step 3, after 30-120 seconds, simultaneously add the remaining dicumyl peroxide and the composite antioxidant; Step 4: After 30-60 seconds, add dioctadecyl thiodipropionate to terminate the reaction.

4. The preparation process according to claim 3, characterized in that, The mass ratio of ethylene-acrylate-maleic anhydride terpolymer, polymethylpentene, dicumyl peroxide, and composite antioxidant is 100:10~30:0.2~0.5:0.2~0.3; The amount of dioctadecyl thiodipropionate added is 0.5 to 1.5 times the total amount of dicumyl peroxide.

5. The preparation process according to claim 3, characterized in that, The compound antioxidant includes antioxidant 1010 and antioxidant 168, with a mass ratio of antioxidant 1010 to antioxidant 168 of 1:1.5~2.

3.

6. The preparation process according to claim 2, characterized in that, In step S100, the core layer granules are obtained by melt blending and extrusion granulation at a temperature of 160~200℃; the surface layer granules are obtained by melt blending and extrusion granulation at a temperature of 180~220℃.

7. The preparation process according to claim 2, characterized in that, In step S200, the surface granules are extruded to form a first surface layer and a second surface layer, and the core layer is disposed between the first surface layer and the second surface layer; the thickness of the first surface layer accounts for 10% to 30% of the total film thickness; the thickness of the second surface layer accounts for 10% to 30% of the total film thickness; and the thickness of the core layer accounts for 40% to 80% of the total film thickness.

8. The preparation process according to claim 2, characterized in that, In S300, cooling includes a rapid cooling stage and a slow cooling stage; the rapid cooling stage uses a cooling medium of 5~20℃ to rapidly cool the film preform, reducing the film preform temperature to 60~80℃, with a cooling time of 1~2 seconds; the slow cooling stage uses a cooling rate of 5~15℃ / second to reduce the film preform temperature from 60~80℃ to 25~40℃. In the initial stage of slow cooling, when the film blank temperature drops to 50~70℃, a longitudinal stretching force of 1.05~1.2 times is applied along the film blank's traveling direction.

9. The preparation process according to claim 2, characterized in that, In S300, the material is first stretched longitudinally with a stretch ratio of 3 to 5 times and a stretching temperature of 90 to 110°C; then stretched laterally with a stretch ratio of 4 to 6 times and a stretching temperature of 100 to 120°C.

10. The preparation process according to claim 2, characterized in that, In S400, the heat setting temperature is 80~100℃ and the heat setting time is 5~15 seconds. After heat setting, the temperature is first reduced to 50~60℃ at a cooling rate of 5~10℃ / second, and then cooled to room temperature at a cooling rate of 15~20℃ / second.

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